In general, the pneumatic radial tire has a carcass, as a framework, extending toroidally between a pair of bead parts, and on the outside in the radial direction of the tire, there is provided a belt layer comprising a rubberized steel cord, as a reinforcing layer. In the pneumatic radial tire as a typical example of rubber article using a steel cord as a reinforcing material, from the viewpoint of improving steering stability and durability, a steel cord that is high in flexibility and superior in durability is demanded as a reinforcing material for the belt layer, and various studies thereof have been hitherto conducted. In particular, important properties necessary for the belt layer to ensure steering stability for a high performance radial tire, are high tensile rigidity in the circumferential direction, high in-plane flexural rigidity, and low out-of-plane flexural rigidity.
That is, the belt member must have high rigidity in the circumferential direction in order to bear a tension by an internal pressure to thereby exert a hoop effect. To this end, firstly, the belt layer preferably has high tensile rigidity in the circumferential direction. Also, since the belt member is subjected to an in-plane bending deformation during cornering, the tire that is smaller in the in-plane bending deformation in the belt creates a larger cornering force, thereby allowing better steering stability to be exercised. Therefore, secondly, the belt layer preferably has high in-plane flexural rigidity.
Furthermore, in the vicinity of cornering limit, the belt layer undergoes a large bending deformation in the in-plane direction. Due to this deformation, inside the bending deformation, the belt layer is subjected to a large compressive deformation, to thereby cause buckling. However, by reducing out-of-plane flexural rigidity of the belt layer, an out-of-plane deformation pressure accompanied with the compression decreases, thereby enabling buckling deformation to be inhibited by the internal pressure of the tire. As a result, a release of a ground contact pressure is suppressed, resulting in a uniform contact pressure. Therefore, thirdly, the belt layer preferably has low out-of-plane flexural rigidity.
In order to reduce the out-of-plane flexural rigidity of the belt layer to thereby improve the steering stability, there are known techniques for applying a steel cord that is small in the diameter of filaments and low in flexural rigidity to the belt. For example, the Patent Document 1 sets forth a technique for improving steering property, stability and the like during cornering by using a specific steel cord composed of filaments with a small diameter (filament diameter: 0.06 to 0.10 mm). The Patent Document 2 discloses a tire of which the steel cord is defined by bending resistance and tensile elongation. The Patent Document 3 discloses a tire having a steel code which is composed of predetermined steel filaments and in which the range of values defined by belt flexural rigidity, cord tenacity, and the void amount in a belt cord is defined to be a predetermined range. The Patent Documents 4 to 6 each disclose a steel cord for tire reinforcement which has a predetermined filament structure and in which the range of values defined by cord tenacity, cord elongation at break, and cord flexural rigidity, is regulated to a predetermined range. Each of these techniques in the Patent Documents 1 to 6 is one that applies a multi-stranded cord using extra-fine steel filaments to the belt layer.
Furthermore, Patent Document 7 discloses a tire (using a steel cord having a 1×n structure composed of 4 or less steel filaments, with the filament diameter being not more than 0.22 mm) wherein the belt is defined by the stranding structure of a belt cord, the filament diameter, and the number of embedded belt cords. Patent Document 8 discloses a tire (using a steel cord having a 1×n structure (n=2 to 5), with the filament diameter being 0.12 to 0.22 mm) that satisfies predetermined requirements of the stranding structure, the flexural rigidity/code tenacity ratio, the code tenacity, and the filament diameter. Patent Document 9 discloses a tire (using a steel cord having a 1×n structure composed of 5 or less steel filaments, with the filament diameter being 0.10 to 0.22 mm) wherein the belt ply of which the belt cord structure and the number of embedded belt cords are defined to be a predetermined ones is arranged via shock-absorbing rubber. Each of these techniques in the Patent Documents 7 to 9 is one which applies a 1×n steel cord using five or less small-diameter steel filaments to the belt layer.
Moreover, steel cords each having a 1×n structure composed of six or more steel filaments stranded in the same direction at the same stranding pitch, are disclosed in Patent Documents 10, 11, and the like.    [Patent Document 1]: Japanese Unexamined Patent Application Publication No. 59-38102 (Claims and others)    [Patent Document 2]: Japanese Unexamined Patent Application Publication No. 60-185602 (Claims and others)    [Patent Document 3]: Japanese Unexamined Patent Application Publication No. 64-85381 (Claims and others)    [Patent Document 4]: Japanese Unexamined Patent Application Publication No. 64-85382 (Claims and others)    [Patent Document 5]: Japanese Unexamined Patent Application Publication No. 64-85383 (Claims and others)    [Patent Document 6]: Japanese Unexamined Patent Application Publication No. 64-85384 (Claims and others)    [Patent Document 7]: Japanese Unexamined Patent Application Publication No. 1-141103 (Claims and others)    [Patent Document 8]: Japanese Unexamined Patent Application Publication No. 3-74206 (Claims and others)    [Patent Document 9]: Japanese Unexamined Patent Application Publication No. 3-143703 (Claims and others)    [Patent Document 10]: Japanese Unexamined Patent Application Publication No. 9-279492 (Claims and others)    [Patent Document 11]: Japanese Unexamined Patent Application Publication No. 9-279493 (Claims and others)